This invention relates to a diffraction grating having a predetermined grating groove pattern, an optical pickup equipped with the diffraction grating and an optical disc device having the optical pickup mounted thereto. More particularly, the invention relates to optical means effective for reproducing information signals from an optical information recording medium (hereinafter called simply an “optical disc”) having multi-layered signal recording layers or recording the information signals to the optical disc.
A multi-layering technology of signal recording layers has been widespread quickly in recent years as means for achieving a large recording capacity of optical discs. The multi-layering technology of the signal recording layers stacks a plurality of signal recording layers with a predetermined layer gap δ among them inside the same optical disc and can substantially increase more than double the recording capacity of single-layered optical discs of the prior art.
To detect highly precisely and stably an information signal from a desired signal recording layer inside such a multi-layered optical disc and a focusing error signal as well as a tracking error signal for converging highly precisely and stably a laser beam to a predetermined recording track of the recording layer, means are essentially necessary for satisfactorily removing the influences of unnecessary reflected beams from recording layers other than the desired signal recording layer (these unnecessary reflected beams from other layers will be hereinafter called “stray optical beams” for short). Several optical technologies and devices as optical means directed to removing the stray optical beams of the multi-layered optical discs have already been disclosed in JP-A-2005-203090, for example.